Yeast cells, like most eukaryotic cells, exhibit cell polarity. The polarized growth of yeast depends upon the localized incorporation of new surface components into the region of the cell known as the bud. This is mediated by the fusion of Golgi-derived secretory vesicles with this specialized region of the plasma membrane. We have defined three systems that work together to mediate the targeting, docking and fusion of secretory vesicles with specific regions of the yeast plasma membrane. Sec2 and the GTP-binding protein Sec4 are needed for the polarized concentration of vesicles at fusion sites. Sec3, Sec 5, Sec 6, Sec 8, Sec 10, Sec 15 and Exo7O comprise the components of a complex, termed the Exocyst, that may specify regions of the plasma membrane that are active in vesicle docking. Sec1 and Sec9 may control the interaction of Snc (an integral protein of.the vesicle) with Sso (an integral protein of the plasma membrane) to complete the transport event We will analyze interactions among the components of each system and between the different systems. In specific: 1) We will use genetic, biochemical and electron microscopic approaches to better define the structure of the Exocyst 2) We will determine which Exocyst subunit mediates membrane attachment, and identify the Exocyst receptor on the plasma membrane 3) We will test the hypothesis that the Exocyst functions to dock vesicles, and that this activity is regulated by the GTP-bound form of Sec4 on the vesicle. 4) We will determine if different sec mutants block the pathway before or after formation of the Snc-Sso complex. 5) We will analyze the phenotype of recessive and dominant SnC mutants and define the functions of their high copy suppressors. 6) We will define the interactions of Sec l with the other components of the exocytic machinery.